fix(understand): a line number is not an equipment tag

parse_tag started a token at the first letter, so '100-P-001', a line
number that begins with its size, read as the pump tag P-001. The review of
the network engine found this. On a P&ID the phantom tag can capture a
valve beside the line as equipment, or collide with the real pump P-001.

A token that is the tail of a word beginning with a digit is no longer a
tag. The same text still yields T101 in 'LINE 100-P-001 TO T101', and
P-001 alone is still a tag. The new test failed before this change; the
label, network, describe, scale, cable and plant-pair suites all pass
(160 tests).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

engineering/understand/labels.py

@@ -229,9 +229,15 @@ def parse_tag(text: str | None) -> str | None:
     ``"Т4100"`` (Cyrillic Т) -> ``"T4100"``; ``"pump M41A"`` -> ``"M41A"``;
     ``"CP-M82"`` stays ``"CP-M82"``. A size label (``DN20``, ``SMS51``), a
     pressure rating (``PN16``) or an IP code (``IP65``) is not a tag; a token
-    that keeps a Cyrillic letter without a Latin twin is not one either.
+    that keeps a Cyrillic letter without a Latin twin is not one either, and
+    neither is the tail of a word that begins with a digit - a line number
+    such as ``100-P-001`` names a pipe, not the pump ``P-001``.
     """
-    for match in _TAG_TOKEN.finditer(plain(text)):
+    source = plain(text)
+    for match in _TAG_TOKEN.finditer(source):
+        word_start = max(source.rfind(" ", 0, match.start()), source.rfind("\n", 0, match.start()))
+        if source[word_start + 1 : match.start()][:1].isdigit():
+            continue
         token = fold_lookalikes(match.group(0)).rstrip(".-")
         if not _TAG_RE.fullmatch(token):
             continue

tests/test_understand_labels.py

@@ -163,6 +163,16 @@ def test_text_without_a_tag():
     assert parse_tag(None) is None
 
 
+def test_a_line_number_is_not_an_equipment_tag():
+    # A line number starts with its size: read as a tag, '100-P-001' would give
+    # 'P-001', which can capture a valve beside the line or collide with a real
+    # pump P-001. A word that begins with a digit holds no tag.
+    assert parse_tag("100-P-001") is None
+    assert parse_tag('2"-P-1001-A1') is None
+    assert parse_tag("LINE 100-P-001 TO T101") == "T101"
+    assert parse_tag("P-001") == "P-001"  # the pump itself is still a tag
+
+
 # -- panels and wiring callouts ----------------------------------------------------